An anesthesia machine 200, as shown schematically in FIGS. 32 and 33, allows gasses to flow through separate inspiratory and expiratory circuits. The anesthesia machine 200 prevents rebreathing of CO2 by absorbing the CO2 in a bed of absorbent material. The anesthesia machine 200 allows rebreathing of some exhaled gases, and the remaining exhaled gases flow through an exhalation 204 valve and through an adjustable pressure limiting valve (pop off valve) 202 and exit the anesthesia machine 200. The adjustable pressure limiting valve 202 is for providing adequate pressure in the anesthesia machine 200, and in a full open position, the adjustable pressure limiting valve maintains a pressure of about 1-3 centimeters (hereinafter cm) of H2O. During manually assisted ventilation, the adjustable pressure limiting valve is left partially open, and during inspiration a breathing bag is squeezed and pressure increases until the adjustable pressure limiting valve relief pressure is reached.
However, it would be desirable if there were a way to build pressure in the anesthesia machine to a predetermined pressure, while eliminating the possibility of over pressurizing the anesthesia machine.